Conventional camera systems are modeled after the single aperture eyes of vertebrates. Single aperture eyes have the advantage of good spatial resolution and good light capture efficiency, but they are disadvantaged by their relatively large size and limited field of view. Vertebrates evolved to overcome the field of view limitation by gimbaling the eye in its socket. Camera developers have overcome the field of view limitation by inventing wide angle lenses modeled after a fish eye. Unfortunately the “fish eye lens” distorts the image, making square objects appear round. The fish eye lens also requires many additional optical elements to control optical aberrations, making the lens expensive, heavy, and long. An example of a wide angle fish eye lens is described by Ogura in U.S. Pat. No. 3,589,798. The lens is over 6 inches long ans uses 10 powered optical elements of different sizes, shapes, and materials.
Another disadvantage of the conventional single aperture lens is its need for focus adjustment to image objects at different distances. The problem is especially acute at close range and has prompted inventors to adopt various schemes to automate the focus adjustment process. One example of an auto focus system is described by Watanabe et al. in U.S. Pat. No. 7,184,090. It engages in a focusing operation while sending out an image-capturing signal. It then settles on the focus position that achieves the highest value in image contrast. As with all such auto focusing schemes, it cannot overcome the inherent design limitation of a single aperture lens: objects at different depths of field cannot be brought into focus simultaneously.
There is the need in many autonomous surveillance, tracking, and navigation applications for a distortion free, wide angle imaging system that remains in focus through all depths of field. Such a system would likely be modeled after the most popular eyes found in nature, the multiple aperture compound eyes of arthropods (i.e. insects and crustaceans). Compound eyes have an extremely wide, undistorted field of view with an infinite depth of field. Compound eyes are distortion free because the field of view and the entrance pupil are separated into tiny zones that are focused independently from each other using an array of micro-lenses (or lenslets). There is no need for focus adjustment because the apertures of these lenslets are extremely small relative to the size of the object and its depth of field. As with a pinhole camera, each aperture captures a very small section of the optical wavefront emanating from the object. The smaller the wavefront section, the flatter it becomes until all objects appear to be at infinity. This is why arthropods have no need for a focusing mechanism.
Compound eyes have the added benefit of being built as convex structures around the outside of the animal's head, making them inherently small and light weight. The ideal artificial compound eye would be conformable to the geometry of its mounting surface and retain the wide angle, focus free, distortion free attributes found in nature. In practice, a conformal artificial compound eye could be shaped to the form of an aircraft wing, nose cone, or fuselage so as not to disrupt the air flowing over it. This would be of great benefit to a micro-aircraft because the artificial compound eye would be part of the aircraft structure, displacing no more volume or mass than necessary. The distortion free, wide angle viewing capability would enable the aircraft to fly autonomously inside buildings, tunnels, and caves using the optical flow field for navigation and guidance. All objects in the flow field would remain in proper focus and shape at all ranges, thereby allowing the aircraft to make real-time avoidance maneuvers.
The concept of an artificial compound eye is not new. Duparré et al. describe a flat lenslet array artificial compound eye the size and shape of a credit card (see Duparré et al., Applied Optics, August 2004, pp 4303-4310, vol. 43, No. 22). The flat design attribute is beneficial in that it enables the use of flat lenslet arrays, which are readily manufactured in a variety of ways (see for example Fadel et al., U.S. Pat. No. 6,967,779). The flat design also matches well to flat mosaic detector arrays, which are also easy to manufacture and readily available. However, the flat design attribute limits the field of view to only 21 degrees in this case. More importantly, the concept is a flat variant of the apposition array compound eyes found in diurnal arthropods (i.e. flies, bees, and butterflies). In an apposition array imaging system the light collection efficiency is equal to the area of the aperture of an individual lenslet. This limits the light collection efficiency considerably, and is why diurnal arthropods require daylight for viewing. Another variation of a flat lenslet array system is described by Gurevich et al. in U.S. Pat. No. 7,187,502. This system uses a second flat lenslet array of a different pitch to increase the magnification of the image. The system was invented for imaging “remotely located objects, i.e., objects located behind the focal distance of the assembly”. Though the flat lenslet arrays described in these inventions are readily available, a curved lenslet array that can be made to any shape would be desirable for making the system conformal to its mounting structure.
Lee and Szema describe an artificial apposition array compound eye that closely mimics the design found in nature (see Lee and Szema, Science, November 2005, pp 1148-1150, vol. 310, No. 5751). The lenslet array is convex in shape, and the light from each lenslet is focused onto a convex surface. Unfortunately, the design requires a convex shaped detector array of extremely small size to capture the image. Even if such a detector array could be made, it would be difficult and expensive to manufacture, and its curvature would need to match the curvature of the lenslet array. The publication by Lee and Szema does not display an image generated by the artificial compound eye they describe, which might be due to problem of finding a proper detector for it. If an image could be captured, its intensity would be very low because it is an apposition array system.
The image intensity can be increased by several orders of magnitude by adopting the more sophisticated superposition compound eye design of nocturnal arthropods, such as moths and beetles. In the superposition compound eye, lenslets are designed to operate as afocal Keplerian telescopes on a convex, meniscus form. This enables thousands of lenslets to work together to bring light to a common point on the convex image capture surface. In the case of a moth eye, its superposition design increases its light collection efficiency by 1000 times that of a butterfly eye, which is why moths are nocturnal and butterflies are diurnal. The challenge then is to create an artificial superposition compound eye that mimics the eye of a moth. In that way its light collection efficiency would rival that of a conventional single aperture imaging system, or even surpass it.
U.S. PATENT DOCUMENTS3,589,798Jun. 29, 1971Ogura6,898,015May 24, 2005Yoshikawa et al.6,933,167Aug. 23, 2005Yamamoto6,942,959Sep. 13, 2005Dubin et al.6,967,779Nov. 22, 2005Fadel et al.7,106,529Sep. 12, 2006Kerr et al.7,119,962Oct. 10, 2006Gurevich et al.7,184,090Feb. 27, 2007Toshimi Watanabe et al.7,187,502Mar. 6, 2007Gurevich et al.